Effect of organic load on the efficacy of activated persulfate in inactivating Escherichia coli O157:H7 and the production of halogenated by-products

Activated persulfate is highly effective in inactivating foodborne pathogens. There is great potential to use this chemical in fresh produce sanitation. The present study evaluated the potential effect of organic load in fresh produce wash water on the bactericidal efficacy of activated persulfate. Romaine lettuce, iceberg lettuce, strawberries, and grapes were used to prepare the produce extracts which were later diluted with water to simulate produce wash water containing different levels of organic matters, represented by the Chemical Oxygen Demand (COD, 100–2000 mg/L). Ferrous activated persulfate (PS/Fe²⁺) and alkaline activated persulfate (PS/OH⁻) were prepared in simulated wash water. A five-strain cocktail of Escherichia coli O157:H7 was added to the activated persulfate solution and treated for 2 min. Water properties, including COD, phenolics, protein content, UV254 absorbance, turbidity, pH, and oxidation reduction potential (ORP), were measured and correlated to the microbial reductions. Trihalomethanes, one of the major types of chlorinated disinfection byproducts, were also monitored. Romaine lettuce inoculated with E. coli O157:H7 was used to validate the organic load effect. The results showed that the organic loads in wash water significantly reduced the effectiveness of bacterial inactivation. The microbial reduction decreased from over 8.0 log CFU/mL to near 2.0 log CFU/mL. The organic load had a significantly larger effect on PS/Fe²⁺ than on PS/OH⁻. Based on the correlation coefficients, COD, phenolics, and UV254 were found to be the major indicators to predict the organic load effect. However, since UV254 measurement was quicker, UV254 was used as the primary indicator for real produce (romaine lettuce) application. When UV254 was adjusted to 0.75 in the simulated wash water, lower reductions of E. coli O157:H7 on romaine lettuce and higher cross-contamination potentials were observed during washing with PS/Fe²⁺, PS/OH⁻, and NaOCl as compared with the lettuce in wash water without additional organic load. In addition, no significant amount of THMs was generated in simulated wash water spiked with produce extracts during activated persulfate treatment. In conclusion, to maintain a high efficacy of activated persulfate for fresh produce washing, the organic load of wash water must be carefully monitored. Activated persulfate is advantageous to traditional chlorine sanitizers as no toxic chlorinated disinfection by-products will be generated.